Recent trends in transportation and industry have placed greater emphasis and demands on air filtration as a means of removing contaminants from the environment. The increase in the proportion of recirculated air in newer commercial passenger planes has raised concerns over increased levels of airborne bacteria and viruses in the working environment of air crews.
It has been well established that viable microorganisms can exist in aerosols and can be spread by aerial transport through air circulating systems. These organisms can remain airborne and viable for considerable lengths of time The survival times will depend on the specific organisms and the relative humidity in the environment. It is reasonable to surmise that a significant percentage of microorganisms in aerosol form will survive through the period of commercial passenger flights.
When considering the air filtration requirements for any application, it will be helpful to have a working knowledge of how air filters work and how they are tested and rated. Air filters usually have to deal with the filtration of particles of from 0.01 micrometers (.mu.m) and up. For reference, a micrometer is equal to 0.000039 inch and the smallest particle which can be seen by the unaided eye is 40 .mu.m,. Typical viruses and the finest cigarette smoke are on the order of 0.01 .mu.m, particle size. The typical droplets in a cloud or fog are 10 .mu.m,.
The three mechanisms by which particles are removed from a gas stream by a filter are direct interception, inertial impaction, and diffusional interception.
Direct interception is the simplest and most easily understood mechanism. The filter medium consists of fiber matrices with defined openings or pores through which the air passes. If the particles in the air are larger than the pores they will be removed by direct interception at the pores.
Filters can also remove particles which are smaller than the filter pore size by inertial impaction. As the air flows around the individual fibers of the filter medium, particles which are of higher density will deviate from the air flow path and impact upon the fibers. The impacted particles adhere to the fibers by forces such as Van der Waals' forces while still being acted upon by forces from the air flow. Larger particles have a higher probability of impaction, but are also subject to larger aerodynamic forces which may overcome the adhesive forces and pull them away from the fibers.
In practice, with considerable variation due to differing particle densities and prevailing flow rates, particles larger than about 0.5 .mu.m, and smaller than about 2-10 .mu.m, will impact and be retained on the fiber surface. Particles smaller than about 0.3 to 0.5 .mu.m, diameter will not impact the fibers and will not be removed efficiently by this mechanism.
The molecules of gases are in constant motion in random directions. Very small particles which are suspended within the gas will be impacted by the moving molecules causing the particles, in turn, to move in a random fashion. Such motion is called "Brownian motion".
Brownian motion will cause small particles to deviate from air stream lines in a manner quite different from the inertial effects described previously for larger particles. These random particle excursions will cause the particles to be collected on the individual filter fibers by the mechanism called diffusional interception. In practice, particles smaller than about 0.1 to 0.3 .mu.m, are efficiently removed by this mechanism.
All gas filters combine all three mechanisms of direct interception, inertial impaction, and diffusional interception to some degree. If the efficiency of removal of a typical filter is measured as a function of particle size, a minimum efficiency will be observed for particles of about 0.3 .mu.m diameter with higher efficiencies for both smaller and larger particles. This behavior results from the following factors
1. Diffusional interception causes high removal for particles below about 0.1 .mu.m, but decreases rapidly for particles which are larger.
2. Below 0.5 .mu.m, removal efficiency due to inertial impaction is low. This combined with the fall off in diffusional interception above 0.1 .mu.m, causes the removal efficiency to be lowest about 0.3 .mu.m,. Generally speaking, efficiency is at a minimum between 0.2 and 0.4 .mu.m.
3. Above 0.5 .mu.m, up to 2-10 .mu.m,, inertial impaction and adhesion is high and the removal efficiency increases.
4. In the size range above 2-10 .mu.m, a typical filter medium provides essentially 100% removal by direct interception.
The removal efficiency of a given filter medium is dependent on several variables including particle size, flow velocity, and medium thickness. The effects of particle size have been discussed. In summary, it can be said that for any given filter medium and flow conditions there is some most-penetrating particle size at which the efficiency is at a minimum. The efficiency will be higher for particle sizes which are larger or smaller than the most-penetrating size.
The effect of flow velocity on particle removal is different depending on the filtration mechanism. An increase in velocity will improve the capture of particles by inertial impaction because the inertial effects which deviate particles from the air flow stream lines are increased. The effect is to extend the regime of this mechanism to smaller particle sizes. In the very small particle size regime where diffusion interception is the predominant removal mechanism, an increase in velocity will decrease removal efficiency. As the residence time in the filter medium is decreased, the probability of interception due to random particle motion is decreased. The net effect of increasing flow velocity through a filter medium will be a decrease in the minimum efficiency and a decrease in the most-penetrating particle size.
The removal efficiency of a filter medium for a given particle size can be increased by increasing the thickness of the filter medium. For example, a filter medium may have a 90% efficiency when challenged by 0.3 .mu.m, particles; that is, 10% of the incident particles pass. If we add a layer of medium, that layer will remove 90% of the incident 10%; that is, 1% of the total will pass. The total efficiency of the two layers is 99%. By adding additional layers the efficiency would become 99.9%, 99.99%, etc. The same effect as adding layers can be achieved by making the medium in a single layer but of multiple thickness. For air filtration, very high efficiencies for 0.3 .mu.m, particles can be achieved using a filter medium with a pore diameter many times larger than 0.3 .mu.m, by making the filter medium with sufficient thickness.
Higher efficiencies can also be achieved by making a filter medium with smaller pores. Smaller pore size enhances the probability of removal by all three mechanisms and for all particle sizes.
The same variables also affect the flow resistance or pressure drop through the filter medium. Increasing the flow velocity and/or medium thickness will also increase the pressure drop through the medium as will decreasing the pore size of the medium. Since the pressure drop which can be tolerated across the filter is limited in most blown air systems, all these variables and their net affects must be considered in designing or selecting a filter for a specific application.
The performance of high efficiency air filters is generally reported as the percentage of influent particles which are removed by the filter. From the above discussions it can be seen that, to be meaningful, the reported efficiency must be related to particle size and flow velocity. When rating or specifying filter media a flow velocity is normally selected for test. This velocity should be within the range in which it would be used in a filter assembly. For assembled filters the efficiency is normally reported at the rated flow of the filter which relates directly to the average velocity over the face of the filter.
Ideally, removal efficiency of a filter would be reported as a continuous function of particle size. This would require costly testing as it is difficult to generate and measure aerosols of a specific size. Another approach is to measure and report the minimum efficiency of a filter for the most-penetrating particle size. This approach has been taken to define the class of filters designated HEPA (High Efficiency Particulate Air). Although the term HEPA is often used to designate any high efficiency air filter, it is specifically defined as an air filter having a minimum efficiency of 99.97% for 0.3 .mu.m, particles of monodispersed dioctylphthalate (DOP). The 0.3 .mu.m, particle size was selected because it is in the range of the generally accepted most-penetrating particle size for filters of this type as discussed previously.
The most definitive way to describe the performance of a filter is to state its removal efficiency for a specific particle size or test aerosol. The test aerosol specified for HEPA filters is monodispersed dioctylphthalate (DOP). This aerosol, sometimes referred to as thermally generated DOP, is formed by condensing DOP oil vapor which has been generated by heating. The resulting aerosol has a mean particle size of 0.3 .mu.m,. In a filter test the aerosol concentration is measured both upstream and downstream of the filter using forward light scattering techniques.
Field or leak testing and some performance testing is conducted using polydispersed or heterogeneous DOP aerosol. This aerosol is generated by passing compressed air through a specifically designed nozzle which has been placed in a DOP oil bath. The resulting aerosol contains a range of particle sizes from less than 0.3 to 3.0 .mu.m, with a mean size at about 0.7 .mu.m,.
In recent years there have been significant innovations in filter design. Initially, filters were not much more than surface filters that had limited dirt capacity before becoming "blinded". These often required significant pressure to force the air through the filter, a pressure that increased significantly with use because of the blinding effect. Of course, increased pressure often led to filter rupture and, even where this was not the case, required powerful impeller motors that were expensive to run and often noisy.
In addition, the filters themselves became bulky because increased throughput could only be achieved by an increase in surface area of the filter.
The development of fiber matrix filters, sometimes called depth filters, greatly increased the dirt capacity of filters. The utility and mode of operation of such filters has been discussed at length above.
The second major development was the construction of filter assemblies in which the filter medium is pleated so that the particulate-containing air contacts a much greater filter surface area for a given unit size. In some devices this idea has been taken further to provide corrugated sheet filters that are then pleated to increase even further the superficial filtering surface presented to the air flow.
One of the most demanding filtration applications has been the filtration of cabin air in airplanes. The design specifications for such filters require a very low pressure drop across the filter, i.e., low resistance to passage of the air, high efficiency of filtration, and very long service intervals. It would be easy to provide a low pressure drop, if lower efficiency would be acceptable. This is not, however, the case. Low pressure drop is nevertheless very important so as to reduce noise and power requirements for the air circulation system. By the same token, it is unacceptable to have relatively dirty air recycled.
Filtration of airplane cabin air is a particularly difficult filtration application since the air often contains significant amounts of fibrous material from carpets, seat fabrics, passenger clothes, and so on. These fibers tend to collect on the upstream surface of a filter and blind the surface of the filter causing an increase in the pressure drop across the filter. With conventional filters it has often proved necessary to install a pre-filter to remove such fibrous matter. This leads to a further maintenance problem in that a mechanic may replace a blinded pre-filter and not recognize that the dirt capacity of the filter itself is close to being reached. As a result, a second maintenance event will be needed within a relatively short time.
The present invention relates to high efficiency air filters having particular design features which permit an unexpectedly small pressure drop across the filter without sacrifice of efficiency or service life. This became possible following the discovery that a fibrous depth filter could be constructed that has the dimensional stability and high filtration efficiency in a very compact form combined with a very low pressure drop across the filter. With this new discovery it has proved possible to design highly efficient air filters capable of meeting the most demanding standards of the aircraft designers while permitting at the same time the elimination of prefilters for the interception of airborne fibers.